Recent developments in wireless communications technologies have allowed expansion of service offerings from the original voice telephone service model to include a number of services supporting packet data communications. As customers become increasingly familiar with data services offered through landline networks, they are increasingly demanding comparable data communications in the wireless domain, for example to maintain service while mobile subscribers roam freely or to provide remote service in locations where wireless loops are preferable to landline subscriber loops. A number of technologies support packet data communications in the wireless domain.
For example, a common-packet channel (CPCH) is an uplink transport channel for transmitting variable size packets from a mobile station (MS) to a base station (BS) without the need for direct resource allocation. The channel resource allocation is contention based. A number of mobile stations could at any time content for the same resources, as found in ALOHA systems. The basic CPCH packet data communication does not address issues such as assignment of the channel resources to various base stations, broadcasting channel status and data rates by the base stations and piggy-backing. If a mobile station cannot access one channel, because the channel is busy, it tries another channel, and so on until it finds and accesses an available channel. When many users are trying to access a limited number of channels, the failed access attempts alone can impose a substantial load and even possibly overload one or more channels of the system.
In a Digital Sense Multiple Access (DSMA) system, whenever a base station detects the presence of a subscriber unit transmission on the reverse channel it asserts a periodically occurring flag, called a “busy/idle” flag, on the associated forward channel. This flag is asserted logically true whenever the channel is busy. Any subscriber unit that already is transmitting when the busy/idle flag is set true may continue to transmit. However, all other subscriber units desiring access to a channel must wait until the busy/idle flag is reset or cleared indicating that at least one channel is idle and available. Since the two-way propagation delay is much less than the minimum packet length, the DSMA type protocols perform much better than traditional slotted ALOHA type protocols. The physical layer and the underlying spread spectrum system allow the quick detection of a collision. This approach only allows the base station to generally throttle back the traffic flow. Essentially, each of the mobile stations will interpret a busy state as an instruction to “backoff” and delay its next access attempt. Although this control from the base station helps to prevent overload, it does not work well when there are multiple CPCH channels. The mobile station(s) cannot determine the busy or idle status of multiple channels from a single busy/idle flag signal.